Photoimageable compositions useful, for example, as photoresists for forming printed circuit boards are taught in U.S. Pat. No. 3,953,309 to Gilano et al., the teachings of which are incorporated herein by reference. The major components of the composition are a photopolymerizable material, e.g., an .alpha.,.beta.-ethylenically unsaturated monomer or short chain oligomer, a photoinitiator chemical system, and an acid functional binder, such as a copolymer of styrene and monobutyl maleate. The photoimageable composition taught in U.S. Pat. No. 3,953,309 also has a free radical inhibitor to prevent premature thermal-initiated polymerization.
The photoimageable compositions taught in U.S. Pat. No. 3,953,309 are developable in alkaline aqueous solution, such as dilute sodium carbonate solution, as a result of significant carboxylic acid functionality of the binder polymer. For example, in a styrene/monobutyl maleate polymer, each monobutyl maleate residue provides one non-esterified carboxylic moiety. A layer of photoimageable composition, subsequent to patterned exposure to actinic radiation and development in alkaline aqueous solution to remove non-exposed portions of the layer, are exposed to an acid etch solution, such as ferric chloride, to etch underlying copper from a circuit board blank.
Photoimageable compositions using as binder polymers of styrene and monoesters of maleic anhydride are also taught in U.S. Pat. Nos. 4,273,857 and 4,293,635, the teachings of each of which are incorporated herein by reference. Binders which are copolymers of styrene and monoesters of maleic anhydride are commercially available, e.g., Scripset.RTM. resins sold by Monsanto Chemical Co.
While the post-development processing in U.S. Pat. No. 3,953,309 is in an acidic solution, other post development processing is conducted in highly alkaline solutions, such as ammoniacal etchants or metal plating solutions. The acid functionality of the polymers in the patents referenced above, while rendering them developable in alkaline aqueous solutions, also render even the exposed, photopolymerized portions of the resist layer subject to degradation in highly alkaline aqueous solutions. In highly alkaline environments, such resists would be subject to delamination and stripping.
U.S. Pat. No. 4,987,054 describes the use of an amine-modified methyl methacrylate/styrene/maleic anhydride "copolymer" which can be thermally cured after exposure and development. This thermal cure produces a film which has excellent resistance to alkaline processing environments. However, the additional post-development thermal curing step can be impractical in high volume printed circuit board production.
U.S. Pat. No. 4,008,087 describes styrene/maleic anhydride copolymers esterified with phenethanol. The polymers are used in silver halide photoemulsions.
U.S. Pat. No. 4,722,947 describes radiation curable polymers which are styrene/maleic anhydride copolymers that are esterified with a hydroxyalkyl acrylyl compound and optionally with another alcohol, such as an arylalkyl monohydric alcohol. The compositions containing the esterified polymer are useful in radiation cured compositions, such as coatings, adhesives and films.
U.S. Pat. No. 4,273,857 describes photoimageable compositions containing styrene/maleic anhydride copolymers partially esterified with methanol and isopropanol.